Some of Britain's most prestigious salmon farmers have been accused of using illegal toxic chemicals, which experts claim destroy the marine environment and endanger human health.

Jackie MacKenzie, 47, a former worker at one of Scotland's longest-established salmon farms on the northwest coast, has blown the whistle on what he calls 'disgusting and dangerous' practices.

The claims will send shock waves through the £260 million Scottish salmon farm industry, which has created thousands of jobs in rural communities but which environmentalists believe has been an ecological disaster.

Despite promotional literature on supermarket shelves showing leaping salmon and idyllic Scottish lochs, critics say the reality of Scottish salmon is thousands of fish stuffed into small pens, fed artifical colouring to make their flesh pink and dosed with chemicals to stop rampant disease.

Although there has been anecdotal evidence of illegal chemicals being used in salmon farming, until now no one involved in the industry has spoken out - because of fears of a backlash from the local community.

But in a signed statement given to The Observer , MacKenzie, who worked at Ardessie Salmon for four years, alleges he was instructed by the company to buy and use Deosect to kill sea lice, parasites that infest intensely farmed salmon.

Deosect is a highly poisonous neurotoxin developed to kill parasites on horses and chicken. It is illegal for use on fish because it can destroy aquatic habitats and is particularly damaging to crustacea such as lobsters and crabs. However, it is much cheaper than the chemical licensed for use in salmon farms.

Deosect contains high concentrations of the controversial pesticide Cypermethrin - which is used in sheep dip and some scientists claim causes sterility and cancer.

MacKenzie says he bought the Deosect from a farm supplier in Inverness and used a false name and paid cash so he could not be traced. He also claims to have bought Deosect for another salmon farmer and says 'the practice is widespread'.

Instructions that come with the pesticide warn it is dangerous to fish and advise the user not to 'contaminate ponds, waterways or ditches'.

A vet at Fort Dodge Animal Health in Southampton, a supplier of Deosect, said: 'As far as marine life goes it is about as toxic as you can get.'

MacKenzie, who has suffered from hepatitis C since a young age, believes using this chemical and another pesticide called Ivermectin exacerbated his health problems.

He said: 'We were spraying this stuff all over the fish without any protective clothing and inhaling it all.' A lot of Ardessie salmon is smoked and sold worldwide.

Following the claims, the Scottish Environment Protection Agency has begun an investigation into the company, which a spokesman said it was 'treating very seriously'. It has already prosecuted a salmon farmer in the Shetland Islands for using Deosect.

Kevin Dunion, director of the Friends of the Earth Scotland, said: 'Salmon farmers appear now to be operating outside the law, with a contempt for both the marine environment and their employees' health and safety. The Scottish Executive must instigate a thoroughgoing review of the industry.'

Today's accusations will severely embarrass Scottish Quality Salmon, the body that regulates the industry and of which Ardessie Salmon is a member. Firms that belong to the organisation can use a tartan kitemark on their salmon as a stamp of its supposed quality.

James Lindsay, chairman of the group, said: 'We will investigate these allegations and take action if they are proved to be true. But the use of illegal chemicals is very rare and this is just one farm out of hundreds.'

Lindsay stressed there was no danger to the public from eating farmed salmon, because tests were done on the fish to ensure there were no chemicals left in them by the time they reached supermarket shelves.

Norman Baker MP, the Liberal Democrats' consumer affairs spokesman, said: 'If these allegations are true, this is all very worrying.

'The industry body claims it is well regulated. But if so, how can one of its members get away with using illegal chemicals until a former employee decides to blow the whistle?

'The Government must undertake random testing of Scottish salmon to ensure this dangerous chemical is not found elsewhere.'

Environmental groups claim a disease that has destroyed large tracts of the Scottish shellfish industry is linked to chemicals used on farmed salmon.

The disease, amnesic shellfish poisoning, can be fatal to humans if they eat infected shellfish. It has plagued Scottish coastal waters for the last decade.

John Parry, owner of Ardessie Salmon, is a trustee of Wester Ross Fisheries Trust, a body set up to protect the marine environment. He refused to deny that the farm had used Deosect.

He said: 'I will not comment on that. There is an ongoing investigation by the Scottish Environment Protection Agency and I await with interest what they will find. We are a small farm and I am utterly amazed that a former employee would want to spread scare stories about the farm.'